Tanks, sewers, and other enclosures that must be entered periodically require some type of air ventilation system for the men working in the enclosure. Without some type of air ventilation the workers would be required to wear respirators. Previously, the ventilation apparatus used normally included an air pump outside the enclosure and an 8-inch flexible hose leading into the enclosure. However, the normal 24 inch (or smaller) manhole is barely large enough to allow a worker to enter the enclosure with tools and/or materials. When an 8-inch ventilating hose is also located within the manhole, it may prevent the worker from entering the enclosure, and provides an obstruction that tends to catch tools on the worker's belt, with the possibility of damaging the hose or dropping tools on another worker already in the enclosure.
A solution to this problem was provided by novel apparatuses and methods described in U.S. Pat. No. 4,794,956 and U.S. Pat. No. 4,982,653, both to Gordon et al, which are specifically incorporated by reference as if reproduced in their entirety herein. The aforementioned patents are assigned to AIR SYSTEMS INTERNATIONAL® of Chesapeake, Va., USA. In one exemplary embodiment, a rigid-walled confined space ventilation conduit comprises a central section having a cross section in the shape of a crescent or a segment of a circle, two intermediate sections attached respectively to each end of the central section, and each having a cross-sectional shape varying from the shape of the central section at the juncture with said central section, and tapering to a circular shape at the outer end of the associated intermediate section. The conduit also includes two outer cylindrical sections, respectively attached to the outer end of each of the intermediate sections, the outer sections being externally aligned on a common axis offset from the center of the central section.
As a result of this construction, it is possible to reduce the cross-sectional obstruction of a relatively small manhole, i.e., with about a 20 inch diameter, to about 10 percent of the cross-sectional area of the manhole, as compared to about 35 percent obstruction for a standard 8 inch diameter hose. For larger manholes, the percent obstruction using the conduit of this invention may be substantially less than 10 percent.
In an exemplary embodiment, an outer surface of the central section is cylindrical and has substantially the same diameter as the diameter of the manhole in which the conduit is used. In the interest of economy, however, it is practical to utilize a standard size conduit which will fit virtually all conventional manholes. For example, a central section having a radius of curvature conforming to the perimeter of a manhole of smaller radius may be effectively utilized in all larger manholes as well.
In a preferred embodiment of the aforementioned invention, the cross-sectional area of the central section may be reduced in comparison to the outer cylindrical sections, but only to the extent of causing a reduction of not more than about 10 percent in air flow rate.
The aforementioned invention also included mounting means at the outer surface of the central section of the conduit so that the conduit may be hung or otherwise attached at a manhole opening.
A related process for using the aforementioned invention in ventilating a confined space via a port includes the steps of providing a rigid-walled confined space ventilation conduit as described above, locating the duct so that one outer end and an associated intermediate section lie outside the enclosure, the other outer end and its associated intermediate section lie inside the enclosure, and the central section extends through the port (e.g., manhole); and operatively connecting the conduit to an external source of air, such as a pump or blower via flexible hosing.
A high quality commercial embodiment of the confined space ventilation conduit described in the aforementioned patents is sold as the SADDLE VENT® confined space ventilator conduit by AIR SYSTEMS INTERNATIONAL®, 821 Juniper Crescent, Chesapeake, Va., 23320, U.S.A. (telephone 800-866-8100).
A typical SADDLE VENT® confined space ventilator conduit produced in the past has been formed of polyethylene. Since polyethylene has very low electrical conductivity—it may be considered an electrical insulator—it allows static electricity to build up on the surface of the device; a static electric charge may also build up on other non-conductive ventilation ducting. Under dry and dusty work conditions the build-up of static electricity can discharge to metal surfaces or other grounded surfaces causing a spark in a work area. Ventilation conduits are often used in petroleum and chemical storage tanks and in municipal sewers that can all contain explosive chemical vapors. Under certain conditions the static build-up on a ventilation duct could lead to an explosion or fire. It is therefore desirable to have a confined space ventilation conduit that is electrically conductive and that is readily able to form an electrical circuit with a grounded source in order to dissipate static electricity and other electric charges. A confined space ventilator conduit is defined herein as a rigidly-walled fluid conduit that has at least a hollow first section having other than a full circle shape in cross section, wherein the conduit can be used to ventilate an enclosure accessed via a port (e.g., a manhole) with less obstruction of the port than if the first section had a hollow full circle cross section of equal area. Exemplary confined space ventilator conduits are described in the aforementioned patents.
Forming confined space ventilator and other ventilation system ducting of metal is not satisfactory for many purposes, as the metal generally does not rebound from dents or crushing forces, and/or can spark when engaging certain surfaces. Further, the raw materials for metal construction can be more expensive than plastic and metal conduits can be much harder to fabricate, particularly a confined space ventilator conduit that has a non-circular cross-section or a rigid-walled elbow joint for a ventilator system. Thus, plastic has been preferred over metal for forming confined space ventilator conduits, such as the SADDLE VENT® confined space ventilator conduit from AIR SYSTEMS INTERNATIONAL®. Although the plastics used are not conductive, they have high mechanical strength, are readily moldable to form a unitary seamless device, and have great durability. The prior art did not recognize and provide a solution for the potential for static electricity buildup on non-conductive confined space ventilator conduits and other respiratory conduits.
Creation of non-metallic electrically conductive respiratory system conduits and in particular a confined space ventilator conduit faced many challenges. Conductive polymers are rare, expensive, and difficult to fabricate, can result in devices with unacceptable mechanical strength, and/or are otherwise impracticable to use. Blending of conductive materials with a suitable polymer faced similar consequences, and/or would result in unacceptable tradeoffs between mechanical strength and durability in order to get a sufficiently conductive product. The prior art does not provide a confined space ventilation system with a continuous electrical connection from the distal end of a flexible hose or conduit inside a confined space, through a confined space ventilator conduit, and to a blower via non-metallic components. While a grounding wire may carry charge past a non-conductive system component, electric charge may still build up on non-conductive components sufficient to create a hazardous condition.
Therefore, objects of this invention are to provide durable and electrically conductive ventilator conduits and an electrically conductive confined space ventilator conduit formed of a polymeric material, and to create processes for using same to ventilate an enclosure via a port into an enclosure and for grounding these components. A further object is to provide a ventilator system incorporating conductive conduits throughout to provide for a continuous electric connection via the length of a confined space ventilator system from a blower and into a confined space. It is another object of this invention to provide a non-metallic electrically conductive confined space ventilator conduit that will not obstruct more than about ten percent of the cross-sectional area of a confined space port (e.g., manhole), without any significant reduction in air flow (e.g., less than about 10% reduction) through all sections of the confined space ventilation conduit and connecting hosing and rigid conduits. Still other objects will become apparent in the more detailed description which follows.
These and other objects of the invention are accomplished by a confined space ventilation conduit (conduit and duct may be used interchangeably herein) formed of an electrically conductive polymer, and having the general confined space ventilator conduit geometry described above. The non-metallic electrically conductive confined space ventilation conduit of the present invention, also referred to herein as a conductive SADDLE VENT® conduit, preferably includes at least one grounding lug for connecting an electrically conductive grounding wire to the conduit, so that an electric charge can be conducted from the conduit to electric ground. In an embodiment, two grounding lugs are provided at opposite ends of the conductive confined space ventilator conduit of the present invention for series connection of the duct into a corresponding grounding circuit. Another embodiment of the present invention is directed to an electrically conductive rigid walled conduit, formed of a non-metallic material, for use in constructing an electrically conductive ventilation system, with a preferred embodiment including a rigid walled electrically conductive ventilation conduit elbow. Preferably, the elbow includes at least one grounding lug. The conductive confined space ventilation conduit of the present invention is preferably designed for serial connection into a ventilation system, and is preferably grounded to a blower forming part of a ventilation system, wherein the blower is electrically grounded.
A preferred ventilation system includes the electrically conductive confined space ventilation duct of the present invention connected to hosing of conventional cylindrical cross-section, with rigid elbows where needed. The other conduits and elbows are preferably formed of an electrically conductive polymer or other electrically conductive material. Grounding lugs may also be formed into or firmly connected to the other electrically conductive ventilation system conduits. In an embodiment, at least one grounding wire is connected serially to the grounding lugs and to electrically conductive components to maintain a complete circuit to ground. Hence, non-conductive ventilation system components can be bypassed to complete the ground circuit, although it is preferred that all hollow components forming the ducting of a ventilation system of the present invention be electrically conductive.
In an embodiment, a conductive coating is applied to non-metallic ventilation system ducting components to provide conductivity. In another embodiment, the present invention includes an electrically conductive, non-metallic conduit for a ventilation system that comprises a rigid conduit formed of a material that is at least electrically dissipative. A preferred material is an ethylene-butene copolymer polyethylene resin with a conductive additive. In one embodiment, the conduit comprises a hollow first section having other than a full circle shape in cross section. In another embodiment, a conductive conduit of the present invention comprises a cylindrical section bent at an approximately ninety degree angle.